Exposure meter



L. M. ARANT EXPOSURE METER Feb. 12, 1946.

Filed Sept. 26, 1944 Inventor zzzmzr/ iflrwzz,

E 25 y W and Patented Feb. 12, 1946 UNITED STATES PATENT OFFICE EXPOSURE METER Luther M. Arant, Southern Pines, N. C. Application September 26, 1944, Serial No. 555,820

' (oi. 23564."l)

1 Claim.

This invention relates to exposure meters for cameras of the type in which the indication of an instrument pointer (such as the pointer moved by the current of a photo-cell) is translated into the data required for the exposure by means of movable scales.

It is an object of the invention to improve excameras showing the scale and the window with the pointer.

Figure 2 is a longitudinal section through the instrument along the line 2-2 of Figure 1, those parts of the indicating instrument which are contained in the casing being removed.

Figure 3 is a, cross section through the instrument along the line 33 of Figure l, the parts in the interior of the instrument being again removed.

The exposure meter according to the invention consists of a casing I0 enclosing the photo-cell, galvanometer coil, resistance, and other parts pertaining to the indicating instruments which are, however, not shown, merely the pointer ll playing on a scale l2 being illustrated in Figure 1. Upon this casing III the disks carrying the scales required for the computation or indication of the settings of the camera controls are mounted.

The annular disk l3 containing the light values and the shutter speeds is the outermost disk encircling the other disks; it is provided with a milled or knurled edge It and may be turned around, using either one of the inner disks or a circular part of the casing as a pivot. In order to be able to seize the annular disk l3 at its periphery with a finger the casing is provided with a depression 15 at both sides over which the knurled edge of the disk I3 protrudes.

Fixedly mounted within the annular disk i3 is the disk 16 carrying the scale indicating the film speed. This disk is screwed to the casing It by means of screws II. The film speed scale occupies only one-half of the disk as shown in Figure l.

A further disk It which is of, semi-circular shape is rotatably mounted on said disk It by means of a pivot 20 fixed to the casing. 'Said disk carries the scale of the aperture stops, in-

dicating the so-called relative sizes 0! the apertures expressed by the ratio between the local distance of the lens and the diameter of the opening used. This scale has a. thumb piece 2i formed by an upturned lug provided on said scale permitting to turn it around the pivot.

The disk I! carries a pointer 22 playing on a scale 23 which contains the filter factor, which represent the ratio of the volume of light passing through a perfectly transparent opening to that passing through the same opening when covered by a filter. The disk indicating the stops to be selected may thus be readjusted in accordance with the absorption of light due to the filter. This readjustment without any additional mental computation will immediately provide the 'necessarydata for setting the controls of the camera.

When operating the exposure meter, thelndications made by the pointer ii on scale l2 which correspond to the prevailing light conditions are translated into the data permitting the setting of the controls of the camera by means of the movable and fixed scales mounted on the ina filter is used, the disk IS with its pointer 22 is turned by means of the thumb piece 2! until the pointer 22 reaches the mark corresponding to the filter factor characterizing the filter used.- Assuming that the filter factor is K2 (the filter factors are usually expressed by the letter K followed by a number indicating the ratio of absorption) the pointer 22 is turned to the mark 2 and the original adjustment will now be corrected and readjusted in conforming with this filter factor, It will now be found, for example, that the aperture stop is f/6.3 and that it is two stops in advance of the stop which would have been selected if no filter had been used.

It will, therefore, be seen that the invention resides mainly in making one of the scales preterably that indicating the aperture stops independently adjustable in accordance with the filter factor, thus taking into account the absorption of lightdue to -the filter.

What I claim is:

In a calculator for use in connection with ex- 2 asaaaae posure meters of the light value indicating type, a supporting base, an outer annular rotatable scale carrying disk provided with an annular light value and an annular shutter speed scale, inner circular portions concentric with the first mentioned disk, divided into two independent sections, one section being fixed to the supporting base and carrying a film emulsion speed scale,

the other section being rotatable and carrying a relative aperture-scale, said last named section being also provided with means for hand operation and with an indicator bridging the surroundlng annular scale, and a fixed filter factor scale arranged on-said supporting base for cooperation with said indicator.

LUTHER M. ARANI. 

